Silylation of porous methylsilsesquioxane films in supercritical carbon dioxide

Microelectronic Engineering 76 (2004) 52–59 www.elsevier.com/locate/mee

Silylation of porous methylsilsesquioxane films in supercritical carbon dioxide Bo Xie, Anthony J Muscat

*

Department of Chemical and Environmental Engineering, University of Arizona, Tucson, AZ 85721, USA Available online 4 August 2004

Abstract Silylation reactions using a supercritical carbon CO2 solvent were performed on porous methylsilsesquioxane (pMSQ) thin films. The addition of alkylsilyl moieties to the films repaired damage due to oxygen plasma processing. The films (JSR LKD 5109) were characterized using Fourier transform infrared (FTIR) spectroscopy, ellipsometry, contact angle, and electrical measurements. The silylation chemistries included <1.5 wt% hexamethyldisilazane (HMDS), tetramethyldisilazane (TMDS), and trimethylchlorosilane (TMCS) mixed with supercritical CO2 (scCO2). Blanket films with a dielectric constant of 2.4 before oxygen ashing and 3.5 after ashing were processed at 150–300 atm and 50–60
C for a 2 min soak time (after a 15 min ramp to steady-state). The disilazanes and chlorosilane reacted with both lone and hydrogen-bonded silanol (SiO–H) groups on the surface of the p-MSQ. The hydrophobicity of the p-MSQ surface was recovered after treatment as shown by contact angles >80
. The dielectric constant of ashed p-MSQ was reduced to 2.5, 3.3, and 2.6 ± 0.1 by HMDS, TMDS, and TMCS addition, respectively. The mechanism proposed involves the direct reaction of the Si-bearing precursor molecules with the p-MSQ surface. Supercritical CO2 is a good solvent for silylation reactions and is an effective approach to restore the degradation of porous MSQ films due to plasma ashing.  2004 Elsevier B.V. All rights reserved. Keywords: Ultra low-k; k-Value restoration; Porous methylsilsesquioxane; Supercritical carbon dioxide; HMDS; TMDS; TMCS

1. Introduction Low-k dielectric films are one of the performance drivers in back end of line integration.

*

Corresponding author. Tel.: +1 520 626 6580. E-mail address: [email protected] (A.J. Muscat).

These films are needed for microelectronic device interconnects to lower power consumption and minimize cross talk between metal lines. Low-k materials currently in production for the 90 nm node are either organics or organosilicates with dielectric constants near 2.8. The second generation of these materials containing manufactured pores filled with air will be needed

0167-9317/$ - see front matter  2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2004.07.028

B. Xie, A.J. Muscat / Microelectronic Engineering 76 (2004) 52–59

to achieve k values below 2.4 for the 45 nm device generation and beyond. Pores compromise the structural integrity of low-k films as well as expose the filmÕs interior to the outside. The open framework creates significant process integration challenges since the pores must be cleaned, free of etching damage, and capped before deposition of the barrier and seed layers. Etching during pattern formation and ashing to remove photoresist damages and chemically modifies low-k layers. Cleaning processes for low-k layers are needed to remove photoresist, deveil features, eliminate contamination trapped in pores, remove copper, and repair patterning damage. Water is introduced into organosilicate low-k layers during ashing of photoresist. The water is present in the form of silanol (Si–OH) groups, which raise the k value typically above 3 and must be removed to achieve low dielectric constants. Photoresist removal is commonly implemented using either O2 or H2/N2 plasma treatments and cleaning steps to remove residual contaminants [1–5]. Wet, dry, and supercritical CO2 (scCO2) cleaning sequences are being investigated for backend applications [6–8]. Wet cleaning utilizes an organic solvent or inorganic acid combined with modifiers such as surfactants, corrosion inhibitors, and complexation agents. Dry cleaning utilizes isotropic plasma. These conventional cleaning approaches, however, face problems with dewetting of nonpolar surfaces, highly porous structures, damage by plasmas, changes in k due to absorption of chemicals, and photoresist poisoning. Cleans based on scCO2 mixed with small amounts of additives such as cosolvents and chelators potentially offer a lower cost solution because of the mass transport, density, tunable solvating power, nonaqueous, low surface tension, reusability and low toxicity of CO2. Several approaches have been used to minimize or repair damage to silicon dioxide containing low-k films after photoresist ashing. The first approach is to use He, H2, or NH3 plasmas to treat the oxygen plasma ashed films and restore the dielectric constant [9,10]. The second approach is to implant boron (B) or arsenic (As) to protect the film during plasma ashing [11,12]. The third ap-

53

proach is to use HMDS or TMCS vapor followed by annealing to remove OH groups and produce a hydrophobic surface [13–17]. Densified fluids heated and compressed to near or above their critical temperature and pressure have solvating properties that are comparable to liquids but mass transfer characteristics comparable to gases making them promising candidates for wafer cleaning applications on both the front and back end of line. scCO2 is being investigated for cleaning porous materials [18], photoresist removal [19], drying to prevent pattern collapse [20,21], deposition of metal layers [22,23] and low-k films [24], etching metals [25] and patterning [26]. Supercritical fluids may be especially useful for processing porous films since they have no surface tension. The goal of this work is to demonstrate viable processes for repair of porous MSQ films using scCO2.

2. Experimental 2.1. Materials The details of the experimental setup have been described previously [27]. P-type (100) orientated silicon wafers with a minimum resistivity of 0.5 ohm cm containing as deposited and O2 ashed blanket porous methylsilsesquioxane (p-MSQ) films (JSR LKD 5109) with a nominal thickness ˚ were supplied by International Semaof 4000 A tech. The silicon wafers were cleaved into 1.5 · 1.5 cm2 pieces for processing and analyzed with FTIR. The silylation chemistries chosen for this study were HMDS [(CH3)3SiNHSi(CH3)3] (97%), TMDS [(CH3)2Si–(H)NHSi–(H)(CH3)2] (97+%), and TMCS [Si(CH3)3Cl] (99+%). The volume percentage of HMDS, TMDS, and TMCS was 1 vol%, which corresponded to 0.35, 0.41, and 0.57 mol%, respectively. All of these chemicals were purchased from Sigma-Aldrich Co. and used as received. The reactor consisted of two stainless steel pieces that screwed together and sealed over a 1 inch diameter seat fitted with a ethylene propylene-90 o-ring (Parker 2–210) in the upper piece producing a total volume of 200 ml. The wafer coupon and silylation chemistry were separated

B. Xie, A.J. Muscat / Microelectronic Engineering 76 (2004) 52–59

in the reactor until it was filled with liquid CO2. An experiment was started by cooling the reactor to 8
C using an ice bath and charging it with liquid CO2 (>99.99%, Air Liquide Coleman grade) to approximately 60 atm. After charging, the reactor was heated and required approximately 12 min to cross into the supercritical CO2 region, and another 3 min to reach steady-state at the desired processing temperature and pressure. The samples were soaked at steady-state conditions for 2 min. After depressurizing and cooling the reactor for approximately two hours, the coupon was removed for post-process analysis. 2.2. Surface analysis and electrical testing

3. Results and discussion 3.1. FTIR spectra One approach to low-k film repair is to use a precursor molecule containing a weak bond to Si. The model compounds using this approach are HMDS and TMDS, which have relatively weak Si–N–Si linkages. Another approach is to chemically react the hydroxyl groups present on the surface after ashing with a molecule that reacts with water, such as the chlorosilane TMCS. Figs. 1–3 show transmission FTIR spectra (a) pre-process, (b) post-process, and (c) difference for 1 vol% HMDS addition to scCO2 at 212 atm and 57
C, TMDS at 202 atm and 53
C, and TMCS at 171 atm and 56
C, respectively. The vibrational stretches are identified in Table 1 [27–30]. The FTIR difference spectra show that HMDS, TMDS, and TMCS reacted with both H-bonded O–H groups (decrease in the O–H peak at 3150– 3560 cm
1) as well as isolated O–H groups (decrease in the O–H peak at 3740 cm
1). There is a concomitant increase in the C–H stretch of CH3 at 2977 and 2922 cm
1 and Si–CH3 peaks at 0.35

0.04

0.30 α1

0.25

γ6 β2 β1

0.20 0.15 0.10

0.03

α2

γ4 γ1

γ5 γ7

γ2 β3 γ3 0.02

(a) (b)

Absorbance

Transmission FTIR spectroscopy (Nicolet Nexus 670 with a MCTA detector) was used ex situ to monitor chemical changes in the low-k films (200 scans at 4 cm
1 resolution co-added for each spectrum, two different spots on all films). All spectra were referenced to a bare silicon background spectrum. The amount of silanol groups reacted was calculated using integrated FTIR peak areas in the range from 3150 to 3560 cm
1 on samples before and after treatment. This range contains the vibrational fingerprint for hydrogenbonded SiO–H stretches. The film thickness of MSQ films was measured using a spectroscopic ellipsometer (J.A. Woollam Co. M-2000). The optical refractive index was modeled in the UV–visible range (247– 725 nm) using a standard Cauchy distribution for a film stack containing a Si substrate and a MSQ film. The hydrophobicity of p-MSQ films was determined by measuring the contact angle of DI water (18.2 MX cm) with a volume of 10 lL using a goniometer (Rame´-Hart Inc. Model 100–00). Metal-oxide-semiconductor capacitors were fabricated for electrical testing by removing oxide from the wafer backside, depositing a substrate contact (100 nm thick Au) and gate metal (100 nm thick and 0.1 cm diameter Au) by e-beam evaporation, and annealing in N2 at 375
C for 30 min. Capacitance–voltage (C–V) curves were measured at 1 MHz with a DC bias of
30 to +30 V at ambient conditions in a light tight box.

The capacitance in accumulation was used to determine the dielectric constant.

Absorbance

54

0.01

0.05 (c) 0.00 4000

0.00 3500

3000

2500

2000

1500

1000

-1

Wavenumber, cm

Fig. 1. Repair of ashed p-MSQ film by addition of 1 vol% HMDS to supercritical CO2. (a) FTIR spectrum of as received ashed p-MSQ; (b) FTIR spectrum after exposing sample in (a) to a 15 min ramp to steady-state conditions and a 2 min scCO2/ 1 vol% HMDS process at 212 atm and 57
C; (c) difference spectrum (b)
(a).

B. Xie, A.J. Muscat / Microelectronic Engineering 76 (2004) 52–59 0.35

0.04 α 1 β3 γ 3 γ2 0.03

0.30 γ6 β2 β1

0.20

0.10

γ γ7

δ2

γ4 γ1

δ1

5

0.02

(a) (b) (c)

Absorbance

Absorbance

0.25

0.15

α2

0.01

0.05 0.00 4000

3500

3000

2500

2000

1500

1000

0.00

-1

Wavenumber, cm

Fig. 2. Repair of ashed p-MSQ film by addition of 1 vol% TMDS to supercritical CO2. (a) FTIR spectrum of as received ashed p-MSQ; (b) FTIR spectrum after exposing sample in (a) to a 15 min ramp to steady-state conditions and a 2 min scCO2/ 1 vol% TMDS process at 202 atm and 53
C; (c) difference spectrum (b)
(a).

0.35

0.04

0.30

Absorbance

0.10

γ5 γ7

γ4 γ1

(a)

0.03

α2

γ2 β3 γ3

0.02

(b)

Absorbance

β2 β1

0.20 0.15

α1

γ6

0.25

0.01

0.05 (c) 0.00 4000

3500

3000 2500 2000 1500 -1 Wavenumber, cm

1000

0.00

Fig. 3. Repair of ashed p-MSQ film by addition of 1 vol% TMCS to supercritical CO2. (a) FTIR spectrum of as received ashed p-MSQ; (b) FTIR spectrum after exposing sample in (a) to a 15 min ramp to steady-state conditions and a 2 min scCO2/ 1 vol% TMCS process at 171 atm and 56
C; (c) difference spectrum (b)
(a).

1279 and 839 cm
1, indicating that chemical moieties with trimethylsilyl –O–Si–(CH3)3 compositions were added to the film by reaction of 50–100% of the surface silanol groups based on integrated FTIR peak areas. For the TMDS process, additional peaks appeared at 2150 cm
1, which was assigned to the Si–H stretch [31], and at 910 cm
1, which was attributed to the Si–H bending mode [32]. The strong band at 1062 cm
1 indicates that the three precursors were cov-

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alently bound to the p-MSQ matrix via surface siloxane Si–O–Si bonds. The lack of a peak at 1650 cm
1 suggests that there were no adsorbed water molecules on the surface of the pores [33]. Comparing difference spectra for the precursors shows that the peaks for the CH3 asymmetric stretch at 2977 cm
1 and CH3 deformation mode at 1279 cm
1 were larger after processing with TMCS/scCO2 than with TMDS/scCO2 or HMDS/scCO2. The decrease in the isolated SiO– H peaks was approximately the same with all three precursors, yet the H-bonded SiO–H peak was smaller for the chlorosilane compared to the disilazanes. The difference FTIR spectra show that the Si–CH3 rocking mode at 839 cm
1 for 1 vol% HMDS/scCO2 and 1 vol% TMCS/scCO2 was shifted to higher wave number. There was no shift of this peak for 1 vol% TMDS/scCO2. Moreover, the Si–CH3 rocking mode vibration was larger for HMDS/scCO2 and TMCS/scCO2 than for the TMDS/scCO2 process, and there was a shoulder at 868 cm
1 on the HMDS and TMCS difference spectra. The Si–CH3 stretch at 779 cm
1 shifted to lower wave number for all silylation processes, but more so for HMDS and TMCS compared to TMDS. In gas and liquid phases, HMDS reacts with the isolated silanol groups but not H-bonded silanol groups on silica surfaces [34–36]. The reaction mechanism proposed proceeds via the physisorption of HMDS on an isolated silanol or geminal silanols, which are two hydroxyls attached to the same Si atom, followed by reaction to produce a surface trimethylsilyl –O–Si–(CH3)3 species and trimethylaminosilane. The intermediate trimethylaminosilane reacts with another silanol producing a second surface trimethylsilyl and ammonia. In supercritical CO2 at 200 bar and 50
C, HMDS was found to proceed via the same reaction pathway as in the gas and liquid phases, except that the ammonia reacted with the CO2 to produce ammonium carbamate [16]. The appearance of FTIR bands at 3510 and 3402 cm
1 after HMDS exposure of fumed silica was attributed to physisorbed ammonium carbamate, which desorbed with prolonged evacuation. These results are consistent with the decrease in the isolated silanol peak at 3740 cm
1, but in contrast to the 70–

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B. Xie, A.J. Muscat / Microelectronic Engineering 76 (2004) 52–59

Table 1 Selected FTIR peak positions Label

Active bond

c (cm
1)

Reference

a1 a2 b1 b2 b3 c1 c2 c3 c4 c5 c6 c7 d1 d2

Si–O–Si stretch (network) Si–O–Si stretch (cage) SiOH  (OH)Si H-bonded stretch SiO–H (isolated/geminal) stretch Si–OH stretch Si–CH3 deformation Si–CH3 rocking Si–CH3 stretch Si–CH3 scissors –CH3 asym stretch –CH3 sym stretch –CH2 stretch Si–H stretch Si–H bend

1062 1118 3150–3560 3740 942 1279 839 779 1409 2977 2922 2875 2150 910

This This This This This This This This This This This This This This

100% decrease in the H-bonded silanol band recorded for HMDS and TMDS treatment of ashed p-MSQ in supercritical CO2 at a similar pressure and temperature (Figs. 1 and 2). The disparity must be due to the surfaces of the fumed silica and ashed porous MSQ film. The H-bonded silanol peak was much larger than the isolated silanol peak on ashed p-MSQ, which is opposite to fumed silica, and molecular water did not physisorb to ashed p-MSQ (Fig. 1(a)) but does to fumed silica [16]. These results suggest that the interaction strength and packing of Hbonded silanol groups on ashed p-MSQ is so that molecular water cannot find sufficient electron density on the surface to form an H-bond. The silanols were created by plasma ashing, which efficiently hydroxylated the top layer of blanket p-MSQ. Extensive H-bonding satisfies the electronegative O atoms and depletes the electropositive H atoms of the silanols such that there was insufficient electron density for another O atom to bond, preventing molecular water from physisorbing. A highly electron deficient H atom, however, is an optimal adsorption site for an N atom, which contains a lone pair of electrons. Donation of electron density from the lone pair on N to form a bond with an H atom on a surface silanol could drive the dissociative adsorption of HMDS and TMDS by ððCH3 Þ3 SiÞ2 NH þ BSiOAH ! BSiOASiðCH3 Þ3 þ ðCH3 Þ3 SiNH2

work, work, work, work, work, work, work, work, work, work, work, work, work, work,

[28–30] [28–30] [28–30] [28–30] [28–30] [28–30] [28–30] [28–30] [28–30] [28–30] [28–30] [30] [31] [32]

silylating the surface. The trimethylaminosilane produced would go on to react with another surface silanol group as proposed in the gas and liquid phase mechanisms, except that reaction with both isolated and H-bonded silanols is possible. The dissociative chemisorption pathway is favored by less bulky groups on the Si bearing precursor, which explains the complete reaction of surface silanols by TMDS. There are two possible silylation reaction pathways for chlorosilanes. Si–halogen bonds have been shown to react with water present in a fluid phase producing trimethylsilanol, which condenses with a surface silanol to deposit a trimethylsilyl – O–Si–(CH3)3 moiety and yield a water molecule [37–39]. Another path is a direct reaction between the chlorosilane and H-bonded silanols on the surface of the film depositing trimethylsilyl. Water in the fluid is catalytic in the first pathway but is not required in the second path. It was not possible to distinguish between these mechanisms since there was water present in the air in the reactor at startup and supercritical CO2 has been shown to extract water from hydrated silica surfaces [17]. The percentage of silanols reacted by TMCS in supercritical CO2 was much larger than that reacted using scCO2 alone [27], which favors the latter mechanism and is supported by the dissociative chemisorption mechanism proposed for the disilazanes.

B. Xie, A.J. Muscat / Microelectronic Engineering 76 (2004) 52–59

57

Table 2 Film thickness, index of refraction, and porosity for p-MSQ p-MSQ

˚) Thickness (A

Index of refraction

Porosity (%)

As-received blanket film As-received ashed film Ashed film after 1 vol% HMDS/scCO2 Ashed film after 1 vol% TMDS/scCO2 Ashed film after 1 vol% TMCS/scCO2

3955 ± 10 3790 ± 10 3754 ± 10 3771 ± 10 3700 ± 10

1.2512 1.2582 1.3019 1.3029 1.3014

37 36 26 26 26

3.2. Ellipsometry Ellipsometry was used to evaluate the thickness and index of refraction after scCO2 processing. Film thickness, index of refraction, and porosity of the p-MSQ films processed with 1 vol% HMDS, TMDS, and TMCS dissolved in scCO2 are shown in Table 2. As-received blanket p-MSQ film had a ˚ . The film thickness was thickness of 3955 ± 10 A ˚ due to oxygen plasma ashreduced to 3790 ± 10 A ing, which converts a portion of the SiCH3 groups into silanol (SiOH) groups in the pores of the film. After processing with scCO2 containing 1 vol% of the Si-bearing precursors, the film thickness was ˚ or <0.3% change in film thickness. 3750 ± 50 A Using the Lorentz-Lorenz equation to calculate

the porosity for p-MSQ films indicates that treatment with the silylation chemistries reduced the film porosity by approximately 10% [40]. Work is in progress to model the ashed p-MSQ using two layers, a top ashing damaged layer and a bottom p-MSQ bulk layer. 3.3. Contact angle and electrical measurements Fig. 4 shows the dielectric constant, contact angle, and percent silanol reacted for different processes. A k value of 2.4 ± 0.1 was measured on MIS devices made using the blanket films from CV curves in accumulation. The contact angle was over 100
indicating that the top surface of the film was hydrophobic. Ashing in an oxygen

Fig. 4. Dielectric constant, contact angle, and percent silanol reacted for as-received blanket and ashed p-MSQ films, and ashed pMSQ films processed with scCO2 containing 1 vol% of HMDS, TMDS, and TMCS.

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B. Xie, A.J. Muscat / Microelectronic Engineering 76 (2004) 52–59

plasma converted a portion of the SiCH3 groups into silanol (SiOH) groups in the pores of the film. The silanol moieties raised the k value to 3.5 ± 0.1 and reduced the contact angle below 10
indicating a hydrophilic surface. By removing 50–100% of the surface silanol groups, the hydrophobicity of the starting surface before ashing was recovered after silylation treatments as confirmed by contact angle measurements of 84–90
and the dielectric constants were reduced to 2.5 ± 0.1 and 2.6 ± 0.1 for HMDS and TMCS, respectively, which are close to the starting value of 2.4 ± 0.1 before ashing. The original k value was recovered even though HMDS and TMCS did not react with all of the silanol groups. The dielectric constant for HMDS dissolved in liquid CO2 at 6
C and 65 atm was 2.9 and at 20
C and 68 atm was 2.8, which shows that the supercritical state is necessary to return the dielectric constant to the starting value. The dielectric constant was reduced to only 3.3 ± 0.1 using TMDS in scCO2, even though all of the silanol groups were reacted and there was no evidence for unreacted precursor remaining in the film, because the Si–H group is polarizable. 4. Conclusions HMDS, TMDS, and TMCS dissolved in scCO2 were used successfully to silylate blanket ashed p-MSQ films producing nearly hydrophobic surfaces. The results are consistent with a direct reaction between the precursors and both isolated and H-bonded silanol groups. The initial dielectric constant of 2.4, which was characteristic of nonashed blanket p-MSQ, was recovered to within 10% using HMDS and TMCS. The results suggest that Si bearing precursors without polarizable chemical groups are an effective approach to repair of oxygen ashed low-k methylsilsesquioxane films using supercritical CO2 and may be useful in capping the pores of ultra low-k dielectrics anticipated to be in production by the 45 nm technology node. Acknowledgement This work was cofunded by International Sematech (306077-OF) and the NSF/SRC Engineering

Research Center for Environmentally Benign Semiconductor Manufacturing (EEC-9528813/2001MC-425). We thank P. Josh Wolf, Steve Burnett, and Eric Busch at International Sematech for program guidance. The authors are grateful for help with contact angle measurements from Professor Srini Raghavan in Materials Science and Engineering and for help with electrical measurement analysis from Professor Hugh Barnaby in the Department of Electrical and Computer Engineering at the University of Arizona. Mentoring from Dr. Philip Matz and Dr. Laura Losey at Texas Instruments was instrumental in the successful completion of this project.

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